Hazardous contaminant emissions have become environmental issues of increasing concern because of the dangers posed to human health. For instance, coal-fired power plants and medical waste incineration are major sources of human activity related mercury emission into the atmosphere. Elemental mercury and its variants, such as methylmercury, are global pollutants.
It has been reported that human inhalation of elemental mercury has acute effects on kidneys and the central nervous system (CNS), such as mild transient proteinuria, acute renal failure, tremors, irritability, insomnia, memory loss, neuromuscular changes, headaches, slowed sensory-motor nerve function, and reduction in cognitive function. Acute inhalation of elemental mercury can affect gastrointestinal and respiratory systems, causing chest pains, dyspnea, cough, pulmonary function impairment, and interstitial pneumonitis. Studies also indicate that chronic exposure to elemental mercury can cause adverse effects on kidneys and the CNS, including erethism (increased excitability), irritability, excessive shyness, insomnia, severe salivation, gingivitis, tremors, and the development of proteinuria.
The main route of human exposure to methylmercury is the diet, such as by eating fish. Acute exposure to methylmercury can cause CNS effects such as blindness, deafness, and impaired level of consciousness. Chronic exposure to methylmercury results in symptoms such as paresthesia (a sensation of prickling on the skin), blurred vision, malaise, speech difficulties, and constriction of the visual field.
It is estimated that there are 48 tons of mercury emitted from coal-fired power plants in the United States annually. One DOE-Energy Information Administration annual energy outlook projected that coal consumption for electricity generation will increase from 976 million tons in 2002 to 1,477 million tons in 2025 as the utilization of coal-fired generation capacity increases. However, mercury emission control regulations have not been rigorously enforced for coal-fired power plants. A major reason is a lack of effective control technologies available at a reasonable cost, especially for elemental mercury control.
A technology currently in use for controlling elemental mercury as well as oxidized mercury is activated carbon injection (ACI). The ACI process involves injecting activated carbon powder into a flue gas stream and using a fabric fiber or electrostatic precipitator to collect the activated carbon powder that has sorbed mercury. ACI technologies generally require a high C:Hg ratio to achieve the desired mercury removal level (>90%), which results in a high portion cost for sorbent material. The high C:Hg ratio indicates that ACI does not utilize the mercury sorption capacity of carbon powder efficiently.
An activated carbon packed bed can reach high mercury removal levels with more effective utilization of sorbent material. However, a typical powder or pellet packed bed has a very high pressure drop, which significantly reduces energy efficiency. Further, these fixed beds are generally an interruptive technology because they require frequent replacement of the sorbent material depending on the sorption capacity.
Activated carbon honeycombs disclosed in US 2007/0261557 may also be utilized to achieve high removal levels of trace contaminants such as toxic metals. A need still exists, however, for more effective utilization of such honeycombs, particularly in the context of system level designs for the removal of trace contaminants such as mercury from fluid streams.
More specifically, coal-fired power plants have limited available space for mercury remediation systems. Power plants also prefer to avoid maintenance of such systems between scheduled shut-downs, which typically occur once per year. A low maintenance, small-size mercury abatement reactor is therefore advantageous.
Although activated carbon honeycomb sorbents with high surface area and abundant sites for mercury sorption are useful, a substantial portion of a reactor comprising these honeycombs is often only partially filled with mercury when used for mercury sorption. For example, in laboratory experiments, only about one-third of the available honeycomb sorbent length in the reactor is essentially saturated after extensive use for mercury removal. The saturated portion of the honeycomb sorbent material in the reactor is located near the inlet of the honeycomb bed, with the remainder downstream often having only a very low concentration of mercury.
The inventors have discovered a method for managing the use of flow-through monolithic sorbents that leads to greater utilization of the sorbent material. Greater utilization of the sorbent material, in turn, leads to a reduction in the total volume of sorbents needed to achieve a desired level of trace contaminant removal, as well as a reduction in the cost of the trace contaminant removal system.